This invention relates to a fingerprint input apparatus and, in particular, to an optical fingerprint input apparatus for acquiring a fingerprint image by the use of a two-dimensional image sensor.
As an optical fingerprint input apparatus using a two-dimensional image sensor, an optical reflective fingerprint input apparatus and an optical transmissive fingerprint input apparatus are known. Recently, attention is directed to the optical transmissive fingerprint input apparatus. In the optical transmissive fingerprint input apparatus, light at first enters into an inside of a finger, is scattered within the inside of the finger, and comes out of the finger through a surface of the finger as outgoing light By the use of the outgoing light, a fingerprint image is formed. In the optical transmissive fingerprint input apparatus, fingerprint identification or collation is more stably and reliably carried out as the light quantity of the outgoing light coming out from the skin of the finger is more uniform over an object surface of the finger for which the fingerprint image is to be acquired. If the light quantity is nonuniform, a greater amount of calculation is required for correction. In this event, it is difficult to reduce the price, the size, and the power consumption of the fingerprint input apparatus. Therefore, it is desired to achieve uniformity in light quantity. To this end, the light is irradiated from an opposite side of the finger opposite to the object surface as a measured surface.
On the other hand, in order to meet the recent demand for reduction in size and thickness of the fingerprint input apparatus, use is made of methods illustrated in FIGS. 1 through 3. Specifically, in the method illustrated in FIG. 1, light from a light source is irradiated to an end of the finger. In the method illustrated in FIGS. 2A and 2B, light from LEDs (light emitting diodes) as a light source is irradiated to lateral sides of the finger. In the method illustrated in FIG. 3, light from LEDs is irradiated to the measured surface of the finger.
In these methods, however, the light quantity in the measured surface is nonuniform. Correction of the nonuniformity in light quantity inevitably requires an increased amount of calculation.
Referring to FIG. 4, description will be made of another method which has been used also. In this method, a fiber optic plate 100 comprising a bundle of a plurality of optical fibers, i.e., an optical fiber bundle, is used in order to acquire a fingerprint image. Each of the optical fibers has a fiber axis inclined at a specific inclination angle with respect to a normal line to a finger contact surface of the fiber optic plate 100. This method often adopts an optical transmissive system as a light irradiation system. Predominantly, the light is irradiated by an illuminating unit located at the opposite side opposite to the measured surface of the finger (FIG. 4) or by the light source located at the lateral sides of the finger (FIGS. 2A and 2B). Rarely, use is made of the method of irradiating the light to the end of the finger as mentioned above in conjunction with FIG. 1.
The method of using the fiber optic plate is also applicable to the optical reflective fingerprint input apparatus. As illustrated in FIG. 5, the light from the light source is irradiated from the lateral side or obliquely from the lateral side of the finger to the optical fiber bundle of the fiber optic plate, guided to the measured surface of the finger, diffusely reflected at fingerprint valleys, and thereafter guided to the optical fiber bundle.
As illustrated in FIG. 6, still another existing method will be described. A light transmitting ring 110 is used to position the finger. The light emitted from the LEDs is irradiated from a lower side or obliquely from the lower side of the ring 110 and injected through the ring 110 into the finger placed on the fiber optic plate 100. The light transmitted through the finger is guided through the optical fiber bundle of the fiber optic plate 100 to a two-dimensional sensor, in this case, the light is guided through the ring 110 as an optical guide to the finger. Thus, the light is irradiated to the finger from the lateral sides thereof.
Referring to FIGS. 7A and 7B, an existing fingerprint input apparatus includes a fiber optic plate 100′ which comprises a combination of illuminating optical fibers for guiding light from LEDs to the finger and measuring optical fibers for guiding light transmitted through the finger to a two-dimensional sensor. As seen from FIG. 7B, the illuminating optical fibers are inclined with respect to the measuring optical fibers.
In a fingerprint input apparatuses using a two-dimensional sensor and having a thin profile (without using a lens or a prism), the two-dimensional sensor is influenced by electrostatic charges accumulated in the finger. If the electrostatic charges have very strong electric energy, the two-dimensional sensor may be broken. In order to avoid the influence by the electrostatic charges, a transparent electrode layer such as tin oxide is formed on the surface of the sensor and grounded in case where the sensor is an optical sensor.
In case where the sensor is a static capacitive sensor, an antistatic electrode can not be applied to its surface. As illustrated in FIGS. 8A and 8B, the static capacitive sensor depicted by a reference numeral 17 is provided with an electroconductive mask 16, relatively wide, formed around a sensing or measuring surface thereof and grounded.
In recent years, attention is directed to the optical fingerprint input apparatus in view of stability and resolution. In addition, following the widespread use of mobile or cellular telephones and the enlargement of the range of use of the mobile telephones beyond telephone conversation, there is an increasing demand for security, in particular, personal authentication. Therefore, it is requested to provide a fingerprint input apparatus which is stably and reliably operable and reduced in size, thickness, power consumption, and price. From the above-mentioned background, the fingerprint input apparatus using the fiber optic plate and the two-dimensional sensor has predominantly been used.
In the fingerprint input apparatus of the type, a fingerprint focusing portion is reduced in thickness. For a light irradiating structure, however, a satisfiable technique is not yet established because of trade-off between the reduction of nonuniformity in light quantity in the measured surface and the reduction in size and thickness. In order to minimize the nonuniformity in light quantity in the measured surface, the best approach is to irradiate the light from the opposite side of the finger opposite to the measured surface, i.e., from the side of a nail. However, this approach requires a space greater than the thickness of the finger and can not be applied to an ultraminiaturized apparatus, such as the mobile telephone, which will be widespread more and more. Thus, from the limitation of the space, it is impossible to use any other method than the light irradiation from the lateral side of the finger or from the side of the measured surface of the finger. However, the light irradiation from the lateral side or from the side of the measured surface of the finger is disadvantageous in the following respects.
For example, consideration will be made of the method described in conjunction with FIG. 6. In this method, the light is irradiated obliquely from the lower side of the finger through the light transmitting ring 110 which serves as a finger position guide. Specifically, the light is emitted from the light source (LEDs) to the air and thereafter enters into the ring 110 as the finger position guide. With this structure, an optical loss is great and, in order to compensate the optical loss, the light source must be increased in brightness. This results in an increase in power consumption. On the other hand, in order to compensate an insufficient light quantity without increasing the power consumption, the light must be irradiated from the close proximity of the measured surface through the ring 110 as the finger position guide. This increases the nonuniformity in light quantity in the measured surface.
In case where the light is irradiated to the end of the finger as illustrated in FIG. 1, the light from the light source often enters directly into the optical fibers facing the measured surface. In addition, the light quantity received at a part of the finger near its base is insufficient.
In the methods described in conjunction with FIGS. 7A and 7B and FIG. 5, the light is irradiated directly to the measured surface of the finger.
In FIGS. 7A and 7B, the illuminating optical fibers for light irradiation are arranged adjacent to the measuring optical fibers for fingerprint measurement and inclined with respect to the measuring optical fibers. The light from the LEDs is irradiated through the illuminating optical fibers to the finger. In this case, the resolution is degraded because of inclusion of the illuminating optical fibers. In addition, the fiber optic plate must be prepared by a combination of the illuminating optical fibers and the measuring optical fibers different in extending direction from each other. This results in an increase in production cost. Therefore, this method does not meet the demand for a low price.
In FIG. 5, the light is irradiated to the finger obliquely from the lower side of the optical fiber bundle of the fiber optic plate. In this case, the illuminating optical fibers are not required. The light is irradiated to the measured surface of the finger through the measuring fibers (including cladding portions). The light scattered in an air layer at the fingerprint valley is measured as a bright part. Thus, this method is basically a measurement of an optical reflective type. On the other hand, external light (upon measurement during the daytime) performs a behavior of an optical transmissive type. Therefore, canceling points are present and result in unstable measurement.
As a technique of a small-sized fingerprint input apparatus presently known, reference will be made to the invention set forth in claim 3 of Japanese Patent No. 3045629. An operation principle similar to the invention is already known. The operation principle is based on the fact that, when the light is directly irradiated to a finger as an object and comes out therefrom after scattered in the finger, the fractions of light coming from a recessed part (valley) and a protruding part (ridge) of the fingerprint are incident into the optical fibers of the fiber optic plate to different extents. Specifically, the fraction of light coming from the protruding part is incident to the optical fiber at a small loss over an aperture angle of the optical fiber because an end of the protruding part is contacted with an end face of the optical fiber. On the other hand, the fraction of light coming from the recessed part enters into the air layer at the valley of the fingerprint, The fraction of light passing through the air layer is reflected at the finger contact surface of the fiber optic plate at a reflectance which is greater as an incident angle is greater with respect to the normal line to the finger contact surface. Therefore, if the optical fiber axis is inclined to an angle at which the reflectance is great and if the fiber has a small numerical aperture, the fraction of light from the air layer hardly enters into the optical fiber. For the protruding part, reflection at a boundary surface is little so that the influence is little. Therefore, the rate of incidence into the optical fiber is different between the fraction of light coming from the protruding part and the fraction of light coming from the recessed part. This brings about a high contrast. The above-mentioned concept is disclosed in U.S. Pat. No. 4,932,776 already published. On the other hand, according to claim 3 in Japanese patent No. 3045629, a critical angle is present in case where the light coming from the air layer at the valley of the fingerprint enters into the optical fiber bundle, and the axis of the optical fiber must be inclined so that the critical angle is beyond the numerical aperture of the optical fiber, i.e., beyond the range of a total reflection critical angle. However, in case where the light is incident from the air layer to the optical fiber having a refractive index greater than that of the air, the critical angle is not present in principle and the reflectance is simply increased with an increase in angle. As shown in FIG. 9, the incident angle θ1 is equal to 85° assuming that the reflectance of 50% corresponds to the critical angle. Therefore, the axis of the optical fiber must be inclined to 85°. In this event, the end face of the optical fiber has an extremely long elliptical shape. The resolution is considerably different in a long axis direction and in a short axis direction. This results in a disadvantage that the resolution in the long axis direction is degraded. Taking the above into consideration, it is necessary in practical applications to seek an optimum condition on the basis of the concept disclosed in U.S. Pat. No. 4,932,776.
In the method of using the fiber optic plate, strong electric energy of the electrostatic charges accumulated in the finger affects the operation of an LSI (Large Scale Integrated Circuit) used as an image sensor. Therefore, the electrostatic charges must be reduced. In the existing technique described above, the measuring surface of the sensor is provided with the transparent electrode layer of a thin film, such as an ITO (Indium Tin Oxide) film. However, the thin film has a large electric resistance so that a large quantity of electrostatic charges can not instantaneously be discharged. In addition, discharge of the electrostatic charges may deteriorate the thin film itself. Furthermore, the thin film is worn as a result of repeated contact with the finger for a long period of time so that the ability of discharging electrostatic charges is degraded. As described in conjunction with FIGS. 8A and 8B, the static capacitive sensor 17 is provided with the electroconductive mask 16, relatively wide, formed around the measuring surface as an antistatic mask because the antistatic electrode such as the ITO film can not be applied to the surface. However, the surface of the static capacitive sensor 17 can not be protected by a relatively thick insulation film or the like. Therefore, if the finger at first comes into close proximity to the sensor before it approaches the antistatic mask, the electrostatic charges are discharged to the sensor to break the sensor.